In drying machines with a rotating drum (e.g. Tumble Dryers) a flow rate of hot air passes through the drum, removing water from wet clothes. Inside the drum, the hot air is cooled down and the heat released by the air allows the evaporation of the water from the clothes. Vented Tumble Dryers suck the ambient air, heat it up by means of an electrical heater and discharge it after flowing in the drum. In condense Tumble Dryers and in heat pump Tumble Dryers, instead, the process air flows in a closed loop: the air is heated up by an electric heater or by the condenser of the heat pump unit before entering the drum and then the air is cooled and dehumidified by an air to air exchanger or by the evaporator of the heat pump unit at drum outlet.
In any case the drying capacity of the process air strictly depends on the heat exchange between the hot air and the wet clothes and can be dramatically affected if occasionally the clothes get wrapped together in one or more knots: in such an event, the inlet hot air cannot remove water efficiently, thus compromising the overall performance of the machine.
In order to prevent the constitution of clothes-knots during the drying cycle, many Tumble Dryers implement special procedures which are launched at fixed time intervals during the cycle, with the purpose to untie possible clothes-knots. As an example, the Tumble Dryer may stop and start the drum rotation or in some cases it can even reverse the drum rotation. It's important to highlight that in any case such procedures introduce specific inefficiencies because they may affect the cycle duration and the drying efficiency (for example if the rotation is stopped for a while the load in the drum cools down and it is necessary to spend energy to warm it up again).
Anyhow, such procedures have several disadvantages, e.g. they are launched at fixed time interval, also when not necessary; therefore they can eventually decrease the overall Tumble Dryer efficiency (cycle time, drying efficiency). Another possible disadvantage can occur in case a clothes-knot is made between one procedure and the next one, the inefficiency introduced by the knot, in this case, would last until the following untangling procedure is started. Furthermore, with prior art systems, even in presence of a clothes-knot, it is not possible to understand if the procedure has been successful or not in unwrapping the knot. It is to be appreciated that the terms “untangling” and “untying” are used interchangeably herein, as are the terms “untangle” and “untie”, and “untangled” and “untied”.
Further, since such untangling procedures are launched in any case, even when there is no need for them, another disadvantage is connected with the electric motor wear due to the many re-starting of the motor.
Additionally, since the untangling procedures are launched at fixed time interval (without being effective and thereby resolving the laundry tangling), uneven dryness of the laundry can still occur.
In most of condensate dryers (with air/air heat exchanger), only one motor drives the process fan and drum and the process fan normally is designed so that its efficiency is high in a process rotation direction, whereas the efficiency is low in a reverse rotation direction and during the untangling procedures the drum and hence the process fan is driven into reverse rotation direction. As a consequence the drying air flow rate is extremely low during the untangling procedures so that the time needed to dry the laundry tends to increase. Please note that during the untangling procedures the electric heater is switched OFF.
As mentioned above, it is known in commercially available systems to perform untangling operations in order to reduce or eliminate the tangling of clothes. One of the problems of such methods is that of establishing the right moment for starting the untangling operations. E.g. U.S. Pat. No. 5,651,194 discloses a method and system for controlling a dryer for drying items in a rotating drum by supplying hot air. The method and system disclosed in U.S. Pat. No. 5,651,194 includes an untangling operation which is launched when a predetermined degree of dryness is reached. The dryness is measured by means of dryness sensors which include electrodes positioned inside the drum so that the items to be dried come into contact with the electrode while the drum is rotating: the degree of dryness is estimated according to the current passing through the electrodes.
The technique discussed in U.S. Pat. No. 5,651,194 does not however address the problem of detecting the clothes tangles. The untangling operation is started anyway when an estimated dryness degree is reached, irrespective of the effective presence of clothes tangling.